Spiral Galaxies with [ITAL]HST[/ITAL]/NICMOS. I. Nuclear Morphologies, Color Maps, and Distinct Nuclei (original) (raw)
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The Astronomical Journal, 2005
We investigate the central regions of 23 spiral galaxies using archival NICMOS imaging and STIS spectroscopy. The sample is taken from our program to determine the masses of central massive black holes (MBH) in 54 nearby spiral galaxies. Stars are likely to contribute significantly to any dynamical central mass concentration that we find in our MBH program and this paper is part of a series to investigate the nuclear properties of these galaxies. We use the Nuker law to fit surface brightness profiles, derived from the NICMOS images, to look for nuclear star clusters and find possible extended sources in 3 of the 23 galaxies studied (13 per cent). The fact that this fraction is lower than that inferred from optical Hubble Space Telescope studies is probably due to the greater spatial resolution of those studies. Using R-H and J-H colors and equivalent widths of Hα emission (from the STIS spectra) we investigate the nature of the stellar population with evolutionary models. Under the assumption of hot stars ionizing the gas, as opposed to a weak AGN, we find that there are young stellar populations (∼10-20 Myr) however these data do not allow us to determine what percentage of the -2total nuclear stellar population they form. Also, in an attempt to find any unknown AGN we use [N II] and [S II] line flux ratios (relative to Hα) and find tentative evidence for weak AGN in NGC 1300 and NGC 4536.
Spiral Galaxies with [ITAL]HST[/ITAL]/NICMOS. II. Isophotal Fits and Nuclear Cusp Slopes
The Astronomical Journal, 2002
We present surface brightness profiles for 56 of the 78 spiral galaxies observed in the HST/NICMOS/Camera-2 F160W snapshot survey introduced in Carollo et al. (2001; paper I), as well as surface brightness profiles for 23 objects out of the 41 that were also observed in the F110W filter. We fit these surface brightness profiles with the Nuker law of Lauer et al. (1995), and use the smooth analytical descriptions of the data to compute the average nuclear stellar cusp slopes γ in the 0.1 ′′-0.5 ′′ radial range. Our main result is the startling similarity between the nuclear stellar cusp slopes γ in the near-infared compared to those derived in the visual passband. This similarity has several implications: (1.) Despite the significant local color variations that are found in the nuclear regions of spirals and that are documented in paper I, there are typically little or no optical-NIR global color gradients, and thus no global stellar population variations, inside ∼ 50-100 pc from the nucleus in nearby spirals. (2.) The large observed range of the strength of the nuclear stellar cusps seen in the HST optical study of spiral galaxies reflects a physical difference between galaxies, and is not an artifact caused by nuclear dust and/or recent star formation. (3.) The dichotomy between R 1/4 bulges, with steep nuclear stellar cusps γ ∼ 1, and exponential bulges, with shallow nuclear stellar cusps γ < 0.3, is also not an artifact of the effects of dust or recent star formation. (4) The presence of a surrounding massive disk appears to have no effect on the rise of the stellar density distribution within the innermost hundred pc of the R 1/4 spheroids. These results imply a breakdown within the family of exponential bulges of the nuclear versus global relationships that have been found for the R 1/4 spheroids. Such a breakdown is likely to have significant implications concerning the formation of exponential bulges and their connection with the R 1/4 spheroids.
Nuclear Properties of a Sample of Nearby Spiral Galaxies from Hubble Space Telescope STIS Imaging
The Astronomical Journal, 2004
We present surface photometry for the central regions of a sample of 48 spiral galaxies (mostly unbarred and barred of type Sbc or Sc) observed with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. Surface brightness profiles (SBPs) were derived and modeled with a Nuker law. We also analyzed archival Wide Field Planetary Camera 2 images with a larger field of view, which are available for 18 galaxies in our sample. We modeled the extracted bulge SBPs with an exponential, an r 1/4 , or an r n profile. In agreement with previous studies, we find that bulges of Sbc galaxies fall into two categories: bulges well described by an exponential profile and those well described by an r 1/4 profile. Only one galaxy requires the use of a more general Sérsic profile to properly describe the bulge. Nuclear photometrically distinct components are found in $55% of the galaxies. For those that we classify as star clusters on the basis of their resolved extent, we find absolute magnitudes that are brighter on average than those previously identified in spiral galaxies. This might be due to a bias in our sample toward star-forming galaxies, combined with a trend for star-forming galaxies to host brighter central clusters.
The ACS Virgo Cluster Survey. VIII. The Nuclei of Early‐Type Galaxies
The Astrophysical Journal Supplement Series, 2006
The ACS Virgo Cluster Survey is a Hubble Space Telescope program to obtain high-resolution imaging, in widely separated bandpasses (F475W ≈ g and F850LP ≈ z), for 100 early-type members of the Virgo Cluster, spanning a range of ≈ 460 in blue luminosity. We use this large, homogenous dataset to examine the innermost structure of these galaxies and to characterize the properties of their compact central nuclei. We present a sharp upward revision in the frequency of nucleation in early-type galaxies brighter than M B ≈ −15 (66 f n 82%) and show that ground-based surveys underestimated the number of nuclei due to surface brightness selection effects, limited sensitivity and poor spatial resolution. We speculate that previously reported claims that nucleated dwarfs are more concentrated to the center of Virgo than their non-nucleated counterparts may be an artifact of these selection effects. There is no clear evidence from the properties of the nuclei, or from the overall incidence of nucleation, for a change at M B ∼ −17.6, the traditional dividing point between dwarf and giant galaxies. There does, however, appear to be a fundamental transition at M B ∼ −20.5, in the sense that the brighter, "core-Sérsic" galaxies lack resolved (stellar) nuclei. A search for nuclei which may be offset from the photocenters of their host galaxies reveals only five candidates with displacements of more than 0. ′′ 5, all of which are in dwarf galaxies. In each case, though, the evidence suggests that these "nuclei" are, in fact, globular clusters projected close to the galaxy photocenter. Working from a sample of 51 galaxies with prominent nuclei, we find a median half-light radius of r h = 4.2 pc, with the sizes of individual nuclei ranging from 62 pc down to ≤ 2 pc (i.e., unresolved in our images) in about a half dozen cases. Excluding these unresolved objects, the nuclei sizes are found to depend on nuclear luminosity according to the relation r h ∝ L 0.50±0.03 . Because the large majority of nuclei are resolved, we can rule out low-level AGN as an explanation for the central luminosity excess in almost all cases. On average, the nuclei are ≈ 3.5 mag brighter than a typical globular cluster. Based on their broadband colors, the nuclei appear to have old to intermediate-age stellar populations. The colors of the nuclei in galaxies fainter than M B ≈ −17.6 are tightly correlated with their luminosities, and less so with the luminosities of their host galaxies, suggesting that their chemical enrichment histories were governed by local or internal factors. Comparing the nuclei to the "nuclear clusters" found in late-type spiral galaxies reveals a close match in terms of size, luminosity and overall frequency. A formation mechanism that is rather insensitive to the detailed properties of the host galaxy is required to explain this ubiquity and homogeneity. The mean of the frequency function for the nucleus-togalaxy luminosity ratio in our nucleated galaxies, log 10 η = −2.49 ± 0.09 dex (σ = 0.59 ± 0.10), is indistinguishable from that of the SBH-to-bulge mass ratio, log 10 (M • /M gal ) = −2.61 ± 0.07 dex (σ = 0.45 ± 0.09), calculated in 23 early-type galaxies with detected supermassive black holes (SBHs). We argue that the compact stellar nuclei found in many of our program galaxies are the low-mass counterparts of the SBHs detected in the bright galaxies. If this interpretation is correct, then one should think in terms of Central Massive Objects -either SBHs or compact stellar nuclei -that accompany the formation of almost all early-type galaxies and contain a mean fraction ≈ 0.3% of the total bulge mass. In this view, SBHs would be the dominant formation mode above M B ≈ −20.5.
Monthly Notices of the Royal Astronomical Society, 2009
We analyse V and H-band surface photometry of a sample of 18 Sb-Sd galaxies. Combining high resolution HST images with ground-based NIR observations, we extract photometric profiles, which cover the whole disk and provide the highest possible resolution. This is the first photometric study of late-type spirals for which the stellar kinematics have been measured. For 10 out of the 18 galaxies, HST data in both F160W (H) and F606W (V ) are available, and, for those, we present colour maps and radial colour profiles at the resolution of the Hubble Space Telescope.
Compact Nuclei in Galaxies at Moderate Redshift: I. Imaging and Spectroscopy
Astrophysical Journal Supplement Series, 1998
This study explores the space density and properties of active galaxies to z=0.8. We have investigated the frequency and nature of unresolved nuclei in galaxies at moderate redshift as indicators of nuclear activity such as Active Galactic Nuclei (AGN) or starbursts. Candidates are selected by fitting imaged galaxies with multi-component models using maximum likelihood estimate techniques to determine the best model fit. We select those galaxies requiring an unresolved, point source component in the galaxy nucleus, in addition to a disk and/or bulge component, to adequately model the galaxy light. We have searched 70 WFPC2 images primarily from the Medium Deep Survey for galaxies containing compact nuclei. In our survey of 1033 galaxies, the fraction containing an unresolved nuclear component greater than 3% of the total galaxy light is 16+/-3% corrected for incompleteness and 9+/-1% for nuclei greater than 5% of the galaxy light. Spectroscopic redshifts have been obtained for 35 of our AGN/starburst candidates and photometric redshifts are estimated to an accuracy of sigma_z=0.1 for the remaining sample. In this paper, the first of two in this series, we present the selected HST imaged galaxies having unresolved nuclei and discuss the selection procedure. We also present the ground-based spectroscopy for these galaxies as well as the photometric redshifts estimated for those galaxies without spectra.
The Astronomical Journal, 2001
We present surface photometry for the central regions of a sample of 33 early-type (E, S0, and S0/a) galaxies observed at 1.6 µm (H band) using the Hubble Space Telescope (HST). Dust absorption has less of an impact on the galaxy morphologies in the near-infrared than found in previous work based on observations at optical wavelengths. When present, dust seems to be most commonly associated with optical line emission. We employ a new technique of two-dimensional fitting to extract quantitative parameters for the bulge light distribution and nuclear point sources, taking into consideration the effects of the point-spread function. Parameterizing the bulge profile with a "Nuker" law , we confirm that the central surface-brightness distributions largely fall into two categories, each of which correlates with the global properties of the galaxies. "Core" galaxies tend to be luminous ellipticals with boxy or pure elliptical isophotes, whereas "power-law" galaxies are preferentially lower luminosity systems with disky isophotes. The infrared surface-brightness profiles are very similar to the optical, with notable exceptions being very dusty objects. Similar to the study of based on optical data, we find that galaxy cores obey a set of fundamental-plane relations wherein more luminous galaxies with higher central stellar velocity dispersions generally possess larger cores with lower surface brightnesses. Unlike most previous studies, however, we do not find a clear gap in the distribution of inner cusp slopes; several objects have inner cusp slopes (0.3 < γ < 0.5) which straddle the regimes conventionally defined for core and power-law type galaxies. The nature of these intermediate objects is unclear. We draw attention to two objects in the sample which appear to be promising cases of galaxies with isothermal cores that are not the brightest members of a cluster. Unresolved nuclear point sources are found in ∼50% of the sample galaxies, roughly independent of profile type, with magnitudes in the range m nuc H = 12.8 to 17.4 mag, which correspond to M nuc H = −12.8 to −18.4 mag. Although the detection rate of compact nuclei seems favored toward galaxies spectroscopically classified as weak active galactic nuclei, we find no significant correlation between the near-infrared nuclear luminosities and either the optical emission-line luminosities or the inferred black-hole masses.
The Astronomical Journal, 2011
We present results of Hubble Space Telescope (HST) NICMOS H-band imaging of 73 of the most luminous (i.e., log[L IR /L ] > 11.4) infrared galaxies (LIRGs) in the Great Observatories All-sky LIRG Survey. This data set combines multi-wavelength imaging and spectroscopic data from space-based (Spitzer, HST, GALEX, and Chandra) and ground-based telescopes. In this paper, we use high-resolution near-infrared data to recover nuclear structure that is obscured by dust at optical wavelengths and measure the evolution in this structure along the merger sequence. A large fraction of all galaxies in our sample possess double nuclei (∼63%) or show evidence for triple nuclei (∼6%). Half of these double nuclei are not visible in the HST B-band images due to dust obscuration. The majority of interacting LIRGs have remaining merger timescales of 0.3-1.3 Gyr, based on the projected nuclear separations and the mass ratio of nuclei. We find that the bulge luminosity surface density L Bulge /R 2 Bulge increases significantly along the merger sequence (primarily due to a decrease of the bulge radius), while the bulge luminosity shows a small increase toward late merger stages. No significant increase of the bulge Sérsic index is found. LIRGs that show no interaction features have on average a significantly larger bulge luminosity, suggesting that non-merging LIRGs have larger bulge masses than merging LIRGs. This may be related to the flux-limited nature of the sample and the fact that mergers can significantly boost the IR luminosity of otherwise low luminosity galaxies. We find that the projected nuclear separation is significantly smaller for ULIRGs (median value of 1.2 kpc) than for LIRGs (median value of 6.7 kpc), suggesting that the LIRG phase appears earlier in mergers than the ULIRG phase.
Monthly Notices of the Royal Astronomical Society, 2000
We present photometry, in V,R,I continuum bands and in the Hα + [NII] emission lines, for a sample of four circumnuclear star forming regions (CNSFR), located in galaxies with different kinds of activity in their nuclei: NGC 7469 (Sy 1), NGC 1068 (Sy 2), NGC 7177 (LINER) and NGC 3310 (Starburst).
Nicmos imaging of infrared-luminous galaxies
2007
We present near-infrared images obtained with the HST NICMOS camera for a sample of 9 luminous (LIGs: L IR (8 − 1000µm) ≥ 10 11 L) and 15 ultra-luminous (ULIGS: L IR ≥ 10 12 L) infrared galaxies. The sample includes representative systems classified as warm (f 25µm /f 60µm > 0.2) and cold (f 25µm /f 60µm ≤ 0.2) based on the mid-infrared colors and systems with nuclear emission lines classified as HII (i.e. starburst), QSO, Seyfert and LINER. The morphologies of the sample galaxies are diverse and provide further support for the idea that they are created by the collision or interactions of spiral galaxies. Although no new nuclei are seen in the NICMOS images, the NICMOS images do reveal new spiral structures, bridges, and circumnuclear star clusters. The colors and the luminosities of the observed clusters are consistent with them being young (10 7−8 yrs), formed as a result of galactic interactions and having masses much greater than those of Galactic globular. In NGC 6090 and VV114, they are preferentially situated along the area of overlap of the two galactic disks. With the exception of IR 17208-0018, all of the ULIGs have at least one compact (2.2µm FWHM ≤ 200 pc) nucleus. Analysis of the near-infrared colors (i.e., m 1.1−1.6 vs. m 1.6−2.2) derived from 1.1 diameter apertures suggests that the warm galaxies have near-infrared colors consistent with QSO+hot dust emission and the cold galaxies, as a group, have near-infrared colors consistent with reddened starlight. In addition, the cold ULIG UGC 5101 (and possibly three others) have near-infrared colors suggesting an additional AGN-like near-infrared component in the nucleus. In a 2 kpc-diameter aperture measurement, the global colors of all of the cold galaxies except UGC 5101 are consistent with starlight with a few magnitudes of visual extinction. The general